Over the years, the performance criteria of gas turbine engines has steadily increased with corresponding improvements in engine efficiency, better thrust-to-weight ratios, lower emissions and improved fuel consumption. However, gas turbine engine temperatures often reach or exceed the limits of the materials of construction, thereby comprising the structural integrity of components in the hot sections of the engine, particularly the gas turbine engine blades. Thus, as gas turbine operating temperatures have increased, various methods have been developed to help protect blades in both the rotor and turbine sections using, for example, high temperature alloys for the combustors and turbine blades. Initially, ceramic thermal barrier coatings (“TBCs”) were applied to the surfaces of components exposed to the hot effluent combustion gases in order to reduce the heat transfer rate and provide thermal protection to the underlying metal and allow components. Such improvements helped to reduce the peak temperatures and thermal gradients of the base metal.
In more recent times, ceramic matrix composites (“CMCs”) were developed as substitutes for many of the high temperature alloys used in conventional turbine engines. CMCs offered improved temperature capability and density advantages over metal blades, often making them the material of choice at the higher anticipated operating temperatures of newer generation turbine engines. A number of new manufacturing techniques have also been developed to produce better quality engine components, particularly the turbine blades, using CMC construction materials. For example, silicon carbide CMCs are now formed from fibrous material infiltrated with molten silicon, such as products made by the “Silcomp” process. Other techniques for forming CMC components include polymer infiltration and pyrolysis (“PIP”) and the slurry cast melt infiltration (“MI”) process. All such processes focus on improving the structural integrity of gas engine components without sacrificing engine performance.
The efforts to develop improved composite rotor blades, stator vanes and airfoils having high strength with elongated filaments composited in a light weight matrix continues to this day. One problem that has discouraged the introduction of new light weight composite gas turbine engine blades is their relative vulnerability to foreign object damage. Many types and sizes of foreign objects can become entrained in the inlet of a gas turbine engine, particularly aircraft engines, ranging from birds to hailstones, sand and dust particles. Turbine damage from foreign objects typically takes two forms. Smaller objects can erode the CMC blade material and eventually reduce the efficiency and degrade the performance of the engine. Any impact by larger objects can rupture or pierce the blades, and portions of an impacted blade can even be torn loose and cause extensive secondary damage to adjacent and downstream blades or other vital engine components. The consequences of foreign object damage appear to be greatest in the low pressure compressors of high bypass gas turbine engines.
Various design improvements have been attempted in an effort to prevent composite blade failures due to foreign objects, such as the inclusion of a protective leading edge blade strip which helps prevent a catastrophic blade failure while providing some erosion protection to the blade, particularly along the leading edge. The edge protection strips allow the energy of impact (due, for example, to a bird strike) to be transmitted down to the trailing edge of the blade. However, even the dissipation of the impact energy can cause the blade to locally oscillate and/or be displaced to a different amplitude, and ultimately fail. Any oscillations or large rapid displacements of the trailing edge also induce strains to the blade matrix which can exceed material system limits and create internal delamination and/or blade surface fracture. Objects impacting a blade can even lead to the loss of edge material and rotor imbalance which in turn limits engine speed and power.
Even though CMC materials are highly resistant to hot temperatures (much more than metals), water vapor in exhaust streams can cause rapid degradation of the matrix, and thus the materials must normally be coated with an environmental barrier coating (“EBC”) in order to protect the underlying matrix from water vapor present in the combustion stream. Unfortunately, the use of thermal coatings on CMC components cannot prevent breaches to the turbine blade itself due to objects impacting against the blades during operation, particularly along the leading edge. Thus, if any penetration of the EBC occurs due to foreign object damage, or by other means such as thermo mechanical shock, the underlying CMC material faces accelerated degradation due to an increased exposure to any water in the hot gas path.
A significant design problem therefore remains in the gas turbine engine field with respect to the use of ceramic matrix composites in the hot gas path. Although adding a barrier coating (EBC) helps to seal the matrix and protect it from hot gas attack, the problem of foreign object damage remains, particularly impacts that penetrate the coating. As detailed below, a new form of ceramic matrix composite blade has been developed as a significant step change in improving the long-term reliability of both blade and engine performance.